Semiconductor device and manufacturing method thereof

ABSTRACT

A device includes an active region, a gate structure, an epitaxial structure, an epitaxial layer, a metal alloy layer, a contact, and a contact etch stop layer. The gate structure is across the active region. The epitaxial structure is above the active region and adjacent the gate structure. The epitaxial layer is above the epitaxial structure. The metal alloy layer is above the epitaxial layer. The contact is above the metal alloy layer. The contact etch stop layer lines sidewalls of the epitaxial structure. The metal alloy layer is spaced apart from the contact etch stop layer.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-23B illustrate a method for manufacturing a semiconductor deviceat various stages in accordance with some embodiments of the presentdisclosure.

FIGS. 24 and 25 are side views of semiconductor devices in accordancewith some embodiments of the present disclosure.

FIGS. 26-36B illustrate a method for manufacturing a semiconductordevice at various stages in accordance with some embodiments of thepresent disclosure.

FIG. 37 is a flow chart of a method M for forming a semiconductor devicein accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As used herein, “around,” “about,” “approximately,” or “substantially”shall generally mean within 20 percent, or within 10 percent, or within5 percent of a given value or range. Numerical quantities given hereinare approximate, meaning that the term “around,” “about,”“approximately,” or “substantially” can be inferred if not expresslystated.

The fins may be patterned by any suitable method. For example, the finsmay be patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers may then be used to pattern thefins.

The gate all around (GAA) transistor structures may be patterned by anysuitable method. For example, the structures may be patterned using oneor more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the GAA structure.

Some embodiments of the present disclosure relate to semiconductordevices having improved epitaxial and metal alloy structures to improvethe contact resistance between source/drain structure and a contactthereon. Although some implementations are illustrated below withregards to FinFETs, it will be appreciated that this concept is notlimited to FinFETs, but is also applicable to other types of devicessuch as MOSFETs, HGAA devices, and the like.

FIGS. 1-23B illustrate a method for manufacturing a semiconductor deviceat various stages in accordance with some embodiments of the presentdisclosure. In some embodiments, the semiconductor device shown in FIGS.1-23B may be intermediate devices fabricated during processing of anintegrated circuit (IC), or a portion thereof, that may include staticrandom access memory (SRAM), logic circuits, passive components and/oractive components, such as p-type field effect transistors (PFETs),n-type FETs (NFETs), multi-gate FETs, metal-oxide semiconductor fieldeffect transistors (MOSFETs), complementary metal-oxide semiconductor(CMOS) transistors, bipolar transistors, high voltage transistors, highfrequency transistors, other memory cells, and combinations thereof.

Reference is made to FIG. 1. A substrate 110 is provided. The substrate110 includes at least one N-type region 110 n and at least one P-typeregion 110 p. At least one N-type device will be formed on the N-typeregion 110 n, and at least one P-type device will be formed on theP-type region 110 p. For ease of explanation, it is assumed that inFIGS. 1-23B, the substrate 110 includes one N-type region 110 n and oneP-type region 110 p adjacent the N-type region 110 n. In someembodiments, the substrate 110 may include silicon (Si). Alternatively,the substrate 110 may include germanium (Ge), silicon germanium (SiGe),gallium arsenide (GaAs) or other appropriate semiconductor materials. Insome embodiments, the substrate 110 may include asemiconductor-on-insulator (SOI) structure such as a buried dielectriclayer. Also alternatively, the substrate 110 may include a burieddielectric layer such as a buried oxide (BOX) layer, such as that formedby a method referred to as separation by implantation of oxygen (SIMOX)technology, wafer bonding, SEG, or another appropriate method. Invarious embodiments, the substrate 110 may include any of a variety ofsubstrate structures and materials.

A plurality of semiconductor fins 112 and 114 are respectively formedover the N-type region 110 n and the P-type region 110 p of thesubstrate 110. The semiconductor fins 112 and 114 may serve as activeregions (e.g., channels and source/drain features) of transistors. It isnoted that the numbers of the semiconductor fins 112 and 114 in FIG. 1are illustrative, and should not limit the claimed scope of the presentdisclosure. In addition, one or more dummy fins may be disposed adjacentboth sides of the semiconductor fins 112 and/or 114 to improve patternfidelity in patterning processes.

The semiconductor fins 112 and 114 may be formed, for example, bypatterning and etching the substrate 110 using photolithographytechniques. In some embodiments, a layer of photoresist material (notshown) is deposited over the substrate 110. The layer of photoresistmaterial is irradiated (exposed) in accordance with a desired pattern(the semiconductor fins 112 and 114 in this case) and developed toremove a portion of the photoresist material. The remaining photoresistmaterial protects the underlying material from subsequent processingoperations, such as etching. It should be noted that other masks, suchas an oxide or silicon nitride mask, may also be used in the etchingprocess. The semiconductor fins 112 and 114 may be made of the samematerial as the substrate 110 and may continuously extend or protrudefrom the substrate 110. The semiconductor fins 112 and 114 may beintrinsic, or appropriately doped with an n-type impurity or a p-typeimpurity.

In some other embodiments, the semiconductor fins 112 and 114 may beepitaxially grown. For example, exposed portions of an underlyingmaterial, such as an exposed portion of the substrate 110, may be usedin an epitaxial process to form the semiconductor fins 112 and 114. Amask may be used to control the shape of the semiconductor fins 112 and114 during the epitaxial growth process.

Reference is made to FIG. 2. Spacing layer material 120′ is deposited onthe exposed sidewalls and top planar surfaces of the semiconductor fins112 and 114. In some embodiments, the spacing layer material 120′ can bemade of dielectric materials. In some embodiments, the spacing layermaterial 120′ may be made of a dielectric material such as, for example,spin-on-glass, silicon nitride, silicon oxynitride, FSG, a low-kdielectric material, and/or other suitable insulating material. In someembodiments, the spacing layer material 120′ is deposited by an ALDprocess. In some embodiments, the deposition of the spacing layermaterial 120′ can be done by suitable processes such as, for example,plasma-enhanced ALD (PEALD), CVD, PVD, molecular beam epitaxy (MBE),high density plasma CVD (HDPCVD), metal organic (MOCVD), remote plasmaCVD (RPCVD), PECVD, other suitable methods, and/or combinations thereof.The spacing layer material 120′ can be deposited between thesemiconductor fins 112 and 114 to form openings 128, in accordance withsome embodiments. By choosing suitable processing deposition parameters,the openings 128 may be configured to create spaces for the subsequentdeposition of self-aligned isolation fins.

In some embodiments, the spacing layer material 120′ includes a firstliner layer 122 and a second liner layer 124. The first liner layer 122is in contact with the substrate 110 and may be a dielectric layer, suchas silicon oxide, silicon nitride, silicon oxynitride, SiCN,SiC_(x)O_(y)N_(z), or combinations thereof. The second liner layer 124is on and in contact with the first liner layer 122 and may be adielectric layer, such as silicon oxide, silicon nitride, siliconoxynitride, SiCN, SiC_(x)O_(y)N_(z), or combinations thereof. The firstliner layer 122 and the second liner layer 124 have different materials.For example, the first liner layer 122 is a nitride layer, and thesecond liner layer 124 is an oxide layer. In some embodiments, the firstliner layer 122 is omitted. In some embodiments, the second liner layer124 is thicker than the first liner layer 122.

Reference is made to FIG. 3. Self-aligned isolation fins 130 are thenformed in the openings 128 (see FIG. 2). In some embodiments, formingthe self-aligned isolation fins 130 includes filling the openings 128with a dielectric fin material. In some embodiments, filling of theopenings 128 may be performed by an ALD process. In some embodiments,the openings 128 may be filled by suitable processes such as, forexample, ALD, CVD, FCVD, PVD, MBE, HDPCVD, MOCVD, RPCVD, PECVD, othersuitable methods, and/or combinations thereof. In some embodiments, thedielectric fin material may be deposited using an FCVD process followedby a subsequent ultra-violet (UV) curing and annealing process. In someembodiments, in-situ doping of carbon and/or nitrogen can be performedto cure or solidify the dielectric fin material during the FCVD process.In some embodiments, the dielectric fin material includes siliconoxynitride (SiON), silicon carbon nitride (SiCN), silicon oxygen carbonnitride (SiOCN), or metal oxides such as, for example, hafnium oxide(HfO₂), zirconium oxide (ZrO₂), aluminum oxide (Al₂O₃), other suitablemetal oxides, and/or combinations thereof. In some embodiments, formingthe self-aligned isolation fins 130 further includes performing aplanarization step (e.g., a CMP step) to remove the excess dielectricmaterials on the upper surfaces of spacing layer material 120′, so thatthe upper surfaces of the self-aligned isolation fins 130 and the uppersurface of spacing layer material 120′ are substantially coplanar. Thedeposition of dielectric fin material forms the self-aligned isolationfins 130 in the openings 128. The self-aligned isolation fins 130 areformed between the semiconductor fins 112 and 114. As the openings 128are defined and formed prior to the deposition of dielectric finmaterial, no alignment process is needed when the dielectric finmaterial fills in the exposed opening 128.

Reference is made to FIG. 4. A planarization process, e.g., a CMPprocess, is performed on the structure of FIG. 3 to expose thesemiconductor fins 112 and 114. The spacing layer material 120′ (seeFIG. 3) is then etched back such that portions of the semiconductor fins112 and 114 and the self-aligned isolation fins 130 protrude from theremaining portions of the spacing layer material 120′. The remainingportions of spacing layer material 120′ forms spacing layer 120. Thespacing layer 120 can be achieved by suitable methods such as, forexample, an etch process that has suitable etch selectivity betweenmaterials of the spacing layer material 120′, the semiconductor fins 112and 114, and the self-aligned isolation fin 130. For example, the etchprocess can have a higher etch rate of the spacing layer material 120′than the etch rate of the semiconductor fins 112 and 114 and/or theself-aligned isolation fin 130. In some embodiments, etching ratedifference be achieved by adjusting suitable parameters of the etchprocess such as, for example, etchant gas type, gas flow rate, etchingtemperature, plasma power, chamber pressure, other suitable parameters,and/or combinations thereof.

Reference is made to FIG. 5. A sacrificial gate dielectric layer 140 isconformally formed above the structure of FIG. 4. In some embodiments,the sacrificial gate dielectric layer 140 may include silicon dioxide,silicon nitride, a high-κ dielectric material or other suitablematerial. In various examples, the sacrificial gate dielectric layer 140may be deposited by an ALD process, a CVD process, a subatmospheric CVD(SACVD) process, a flowable CVD process, a PVD process, or othersuitable process. By way of example, the sacrificial gate dielectriclayer 140 may be used to prevent damage to the semiconductor fins 112and 114 by subsequent processing (e.g., subsequent formation of thedummy gate structure).

Subsequently, at least one dummy gate structure 150 is formed above thesacrificial gate dielectric layer 140. The dummy gate structure 150includes a dummy gate layer 152, a pad layer 154 formed over the dummygate layer 152, and a mask layer 156 formed over the pad layer 154.Formation of the dummy gate structure 150 includes depositing insequence a dummy gate layer, a pad layer and a mask layer over thesubstrate 110, patterning the pad layer and mask layer into patternedpad layer 154 and mask layer 156 using suitable photolithography andetching techniques, followed by patterning the dummy gate layer usingthe pad layer 154 and the mask layer 156 as masks to form the patterneddummy gate layer 152. As such, the dummy gate layer 152, the pad layer154, and the mask layer 156 are referred to as the dummy gate structure150. In some embodiments, the dummy gate layer 152 may be made ofpolycrystalline-silicon (poly-Si), poly-crystalline silicon-germanium(poly-SiGe), or other suitable materials. The pad layer 154 may be madeof silicon nitride or other suitable materials, and the mask layer 156may be made of silicon dioxide or other suitable materials.

Reference is made to FIG. 6. The sacrificial gate dielectric layer 140of FIG. 5 is patterned using the dummy gate structure 150 as an etchingmask. Subsequently, blanket layers 162′ and 164′ of insulating materialsfor gate spacers 160 a (see FIG. 7) are conformally formed on thestructure of FIG. 5 by using plasma enhanced chemical vapor deposition(PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmosphericchemical vapor deposition (SACVD), or the like. The blanket layers 162′and 164′ are deposited in a conformal manner so that it is formed tohave substantially equal thicknesses on vertical surfaces, such as thesidewalls, horizontal surfaces, and the top of the dummy gate structure150. In some embodiments, the insulating material of the blanket layers162′ and 164′ are silicon nitride-based materials, such as SiN, SiON,SiOCN or SiCN and combinations thereof.

Reference is made to FIG. 7. The blanket layers 162′ and 164′ (see FIG.6) are then etched using an anisotropic process to form gate spacers 160a on opposite sidewalls of the dummy gate structure 150 and fin sidewallspacers 160 b on opposite sidewalls of the semiconductor fins 112 and114. In some embodiments, the gate spacers 160 a and the fin sidewallspacers 160 b include a seal spacer 162 a, 162 b and a main spacer 164a, 164 b. The seal spacers 162 a (162 b) may be formed on sidewalls ofthe dummy gate structure 150 (sidewalls of the semiconductor fins 112and 114) and the main spacers 164 a and 164 b are formed on the sealspacers 162 a and 162 b. In some embodiments, the anisotropic processcan be controlled such that no fin sidewall spacers 160 b remain on thespacing layer 120. The anisotropic etching performed on the blanketlayers 162′ and 164′ can be, for example, reactive ion etching (RIE).During the anisotropic etching process, most of the insulating materialis removed from horizontal surfaces, leaving the dielectric spacer layeron the vertical surfaces such as the sidewalls of the dummy gatestructures 150 and the sidewalls of the exposed semiconductor fins 112and 114.

Reference is made to FIG. 8. A plurality of recesses 118 are formed onopposite sides of the dummy gate structure 150 by etching thesemiconductor fins 112 and 114. The dummy gate structure 150 and thegate spacers 160 a act as etching masks in the formation of the recesses118. The etching process includes a dry etching process, a wet etchingprocess, or combinations thereof.

Semiconductor materials are then deposited in the recesses 118 to formepitaxial structures 170 and 180 which are referred to as source/drainregions. The epitaxial structures 170 are form above the N-type region110 n, and the epitaxial structures 180 are respectively form above theP-type region 110 p. The epitaxial structures 170 and 180 mayalternatively be referred to as raised source and drain regions. Thesemiconductor materials include a single element semiconductor material,such as germanium (Ge) or silicon (Si), compound semiconductormaterials, such as gallium arsenide (GaAs), silicon arsenide (SiAs), oraluminum gallium arsenide (AlGaAs), or a semiconductor alloy, such assilicon germanium (SiGe), silicon germanium boron (SiGeB), or galliumarsenide phosphide (GaAsP). The epitaxial structures 170 and 180 havesuitable crystallographic orientations (e.g., a (100), (110), or (111)crystallographic orientation). In some embodiments, the epitaxialstructures 170 and 180 include source/drain epitaxial structures. Insome embodiments, where an N-type device is desired, the epitaxialstructures 170 may include an epitaxially grown silicon phosphorus (SiP)or silicon carbon (SiC). In some embodiments, where a P-type device isdesired, the epitaxial structures 180 may include an epitaxially grownsilicon germanium (SiGe). The epitaxial processes include CVD depositiontechniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD(UHV-CVD)), molecular beam epitaxy, and/or other suitable processes.Desired p-type or n-type impurities may be, or may not be, doped whilethe epitaxial process. The doping may be achieved by an ion implantationprocess, plasma immersion ion implantation (PIII) process, gas and/orsolid source diffusion process, other suitable process, or combinationsthereof.

In some embodiments, the epitaxial structure 170 includes a firstepitaxial layer 172 and a second epitaxial layer 174. The firstepitaxial layer 172 is in direct contact with the recessed portion ofthe semiconductor fin 112, and the second epitaxial layer 174 is abovethe first epitaxial layer 172. The first and second epitaxial layers 172and 174 are crystalline semiconductor layers, such as Si, SiC, SiCP, andSiP, having different lattice constants from each other and from thesemiconductor fin 112. When SiC, SiP and/or SiCP are used, the C or Pconcentration of the first epitaxial layer 172 is different from that ofthe second epitaxial layer 174. In some embodiments, a Group III-Vsemiconductor layer is used for at least one of the first and secondepitaxial layers 172 and 174. In some other embodiments, only one or twoof the first and second epitaxial layers 172 and 174 is formed, and insome other embodiments, more epitaxial layers are formed. For example, athird epitaxial structure may be formed above and wrap around the secondepitaxial layer 174.

Similarly, the epitaxial structure 180 may include a first epitaxiallayer 182 and a second epitaxial layer 184. The first epitaxial layer182 is in direct contact with the recessed portion of the semiconductorfin 114, and the second epitaxial layer 184 is above the first epitaxiallayer 182. The first and second epitaxial layers 182 and 184 arecrystalline semiconductor layers, such as Si, Ge, and SiGe, havingdifferent lattice constants from each other and from the semiconductorfin 114. When Si, Ge, and SiGe are used, the Ge atomic concentration ofthe first epitaxial layer 182 is different from that of the secondepitaxial layer 184. In some embodiments, a Group III-V semiconductorlayer is used for at least one of the first and second epitaxial layers182 and 184. In some other embodiments, only one or two of the first andsecond epitaxial layers 182 and 184 is formed, and in some otherembodiments, more epitaxial layers are formed. For example, a thirdepitaxial structure may be formed above and wrap around the secondepitaxial layer 184.

Reference is made to FIG. 9. A contact etch stop layer (CESL) 190 isconformally formed over the structure of FIG. 8. In some embodiments,the CESL 190 can be a stressed layer or layers. In some embodiments, theCESL 190 has a tensile stress and is formed of Si₃N₄. In some otherembodiments, the CESL 190 includes materials such as oxynitrides. In yetsome other embodiments, the CESL 190 may have a composite structureincluding a plurality of layers, such as a silicon nitride layeroverlying a silicon oxide layer. The CESL 190 can be formed using plasmaenhanced CVD (PECVD), however, other suitable methods, such as lowpressure CVD (LPCVD), atomic layer deposition (ALD), and the like, canalso be used. In some embodiments, at least one bottom air gap 192 maybe formed under the epitaxial structures 170 and/or 180 and defined bythe CESL 190.

An interlayer dielectric (ILD) 195 is then formed on the CESL 190. TheILD 195 may be formed by chemical vapor deposition (CVD), high-densityplasma CVD, spin-on, sputtering, or other suitable methods. In someembodiments, the ILD 195 includes silicon oxide. In some otherembodiments, the ILD 195 may include silicon oxy-nitride, siliconnitride, compounds including Si, O, C and/or H (e.g., silicon oxide,SiCOH and SiOC), a low-k material, or organic materials (e.g.,polymers). After the ILD 195 is formed, a planarization operation, suchas CMP, is performed, so that the pad layer 154 and the mask layer 156(see FIG. 8) are removed and the dummy gate layer 152 is exposed.

Reference is made to FIG. 10. Subsequently and optionally, a replacementgate (RPG) process scheme is employed. In the RPG process scheme, adummy polysilicon gate (the dummy gate structure 150 in FIG. 8 in thiscase) is formed in advance and is replaced later by a metal gate. Insome embodiments, the dummy gate structure 150 is removed to form a gatetrench 158 with the gate spacers 160 a as its sidewalls. In some otherembodiments, the sacrificial gate dielectric layer 140 (see FIG. 9) isremoved as well. The dummy gate structure 150 (and the sacrificial gatedielectric layer 140) may be removed by dry etching, wet etching, or acombination of dry and wet etching.

A gate dielectric layer 212 is formed in the gate trench 158, and atleast one metal layer is formed in the gate trench 158 and on the gatedielectric layer 212. Subsequently, a chemical mechanical planarization(CMP) process is performed to planarize the metal layer and the gatedielectric layer 212 to form a metal gate structure 210 in the gatetrench 158. The metal gate structure 210 across the semiconductor fins112 and 114. The metal gate structure 210 includes the gate dielectriclayer 212 and a metal gate electrode 214 over the gate dielectric layer212. In some embodiments, the gate dielectric layer 212 includes ahigh-k material (k is greater than 7) such as hafnium oxide (HfO₂),zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), hafnium aluminum oxide(HfAlO₂), hafnium silicon oxide (HfSiO₂), aluminum oxide (Al₂O₃), orother suitable materials. In some embodiments, the gate dielectric layer212 may be formed by performing an ALD process or other suitableprocess. The metal gate electrode 214 may include metal layers 215,e.g., work function metal layer(s) and capping layer(s), a fill layer(s)216, and/or other suitable layers that are desirable in a metal gatestack. The work function metal layer 215 may include an n-type and/or ap-type work function metal. Exemplary n-type work function metalsinclude Ti, Ag, TaAl, TaAlC, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, othersuitable n-type work function materials, or combinations thereof.Exemplary p-type work function metals include TiN, TaN, Ru, Mo, Al, WN,ZrSi₂, MoSi₂, TaSi₂, NiSi₂, WN, other suitable p-type work functionmaterials, or combinations thereof. The work function metal layer 215may have multiple layers. The work function metal layer(s) 215 may bedeposited by CVD, PVD, electroplating and/or other suitable processes.In some embodiments, the capping layer may include refractory metals andtheir nitrides (e.g., TiN, TaN, W₂N, TiSiN, and TaSiN). The cappinglayer may be deposited by PVD, CVD, metal-organic chemical vapordeposition (MOCVD) ALD, or the like. In some embodiments, the fill layer216 may include tungsten (W). The fill layer 216 may be deposited byALD, PVD, CVD, or another suitable process.

Reference is made to FIGS. 11A and 11B, where FIG. 11B is a side view ofthe semiconductor device in FIG. 11A. The ILD 195 and the CESL 190 arethen etched to form openings 197 and 198 by various methods, including adry etch, a wet etch, or a combination of dry etch and wet etch. Theopenings 197 and 198 extend substantially vertically through the ILD 195and the CESL 190. The opening 197 is formed above the epitaxialstructure 170 and exposes the epitaxial structure 170, and the opening198 is formed above the epitaxial structure 180 and exposes theepitaxial structure 180. While the etching process, portions of theepitaxial structures 170 and 180 are removed, such the top surfaces 171and 181 of the epitaxial structures 170 and 180 are concave.

Reference is made to FIG. 12. A first mask layer 220 is conformallyformed above the structure of FIG. 11B. That is, the first mask layer220 lines the inner sidewalls of the CESL 190 and ILD 195 and the topsurfaces 171 and 181 of the epitaxial structures 170 and 180. In someembodiments, the first mask layer 220 is a dielectric layer such as ametal oxide layer. In some embodiments, the first mask layer 220includes a high-k material (k is greater than 7) such as aluminum oxide(Al₂O₃), zirconium oxide (ZrO₂, Zr₂O₃), hafnium oxide (HfO₂), lanthanumoxide (La₂O₃), hafnium aluminum oxide (HfAlO₂), hafnium silicon oxide(HfSiO₂), or other suitable materials. In some embodiments, the firstmask layer 220 may be formed by performing an ALD process or othersuitable process. In some embodiments, the first mask layer 220 has athickness T1 in a range of about 3 nm to about 5 nm. If the thickness T1of the first mask layer 220 is greater than about 5 nm, the depositionwindow for the following formed epitaxial layer 240 (see FIG. 15) may besmall; if the thickness T1 of the first mask layer 220 is less thanabout 3 nm, the first mask layer 220 may be completely removed duringthe surface cleaning process described in FIG. 15, and the top surface171 of the epitaxial structure 170 is exposed.

Subsequently, a second mask layer 230 is optionally formed above thefirst mask layer 220. The first and second mask layers 220 and 230include different materials. For example, the second mask layer 230 isfree from metal. The second mask layer 230 may be a low-k dielectriclayer, such as silicon oxide, silicon nitride, silicon oxynitride, orcombinations thereof. In some embodiments, the second mask layer 230 maybe formed by performing an ALD process or other suitable process. Insome embodiments, the second mask layer 230 has a thickness T2 in arange of about 3 nm to about 5 nm. If the thickness T2 of the secondmask layer 230 is greater than about 5 nm, the deposition window for thefollowing formed epitaxial layer 240 (see FIG. 15) may be small; if thethickness T2 of the second mask layer 230 is less than about 3 nm, thesecond mask layer 230 may not protect the first mask layer 220 duringthe following surface cleaning process as described in FIG. 15.

Reference is made to FIG. 13. A first photoresist layer 310 is formedabove the N-type region 110 n of the substrate 110. The firstphotoresist layer 310 is formed by spin-on coating or other suitabletechnique. Other operations, such as baking, may follow the coating ofthe first photoresist layer 310. The first photoresist layer 310 exposesa portion of the second mask layer 230 above the P-type region 110 p.

Subsequently, a directional etching process is performed on the portionof the second mask layer 230 above the P-type region 110 p to remove thehorizontal portions of the second mask layer 230. The directionaletching process may have suitable etch selectivity between materials ofthe first and second mask layers 220 and 230. For example, the etchprocess can have a higher etch rate of the second mask layer 230 thanthe etch rate of the first mask layer 220. In some embodiments, theetching selectivity of the first mask layer 220 to the second mask layer230 is greater than about 10, e.g., in a range of about 10 to about 50.In some embodiments, etching rate difference be achieved by adjustingsuitable parameters of the etch process such as, for example, etchantgas type, gas flow rate, etching temperature, plasma power, chamberpressure, other suitable parameters, and/or combinations thereof. Theremaining vertical portions of the second mask layer 230 above theP-type region 110 p form first spacers 232 on the sidewalls of theopening 198. Further, a portion of the first mask layer 220 above thetop surface 181 of the epitaxial structure 180 is exposed by the firstspacers 232.

Reference is made to FIG. 14. After the formation of the first spacers232, the first photoresist layer 310 (see FIG. 13) is removed from theN-type region 110 n of the substrate 110 by an appropriate process, suchas etching or ashing process. In some embodiments, the second mask layer230 is configured to be a protection layer for protecting the first masklayer 220 from being removed during the removal of the first photoresistlayer 310. In some other embodiments, the second mask layer 230 can beomitted if the removal process us an etching process that has suitableetch selectivity between materials of the first mask layer 220 and thefirst photoresist layer 310.

Subsequently, a portion of the first mask layer 220 exposed by thesecond mask layer 230 or the first spacers 232 is removed by using thesecond mask layer 230 and the first spacers 232 as etching masks. Theremoval process may be an etching process that has suitable etchselectivity between materials of the first and second mask layers 220and 230 and the ILD 195. For example, the etch process can have a higheretch rate of the first mask layer 220 than the etch rate of the secondmask layer 230 and the etch rate of ILD 195. As such, the etch processdoes not damage the ILD 195 (and the CESL 190). In some embodiments,etching rate difference be achieved by adjusting suitable parameters ofthe etch process such as, for example, etchant gas type, gas flow rate,etching temperature, plasma power, chamber pressure, other suitableparameters, and/or combinations thereof. As such, the top surface 181 ofthe epitaxial structure 180 is exposed by the first mask layer 220.Further, the remaining vertical portions of the first mask layer 220above the P-type region 110 p form second spacers 222 on the sidewallsof the opening 198.

Reference is made to FIG. 15. A surface cleaning process is performed onthe structure of FIG. 14 for removing native oxides on the top surface181 of the epitaxial structure 180. The second mask layer 230 and thefirst spacers 232 (see FIG. 14) are also removed during this surfacecleaning process. As such, the remaining first mask layer 220 and thesecond spacers 222 are exposed. In some embodiments, the surfacecleaning process removes portions of the remaining first mask layer 220and the second spacers 222, such that the thicknesses of the remainingfirst mask layer 220 and the second spacers 222 are reduced. In someother embodiments, the second mask layer 230 can be omitted if thesurface cleaning process barely removes the first mask layer 220 and thesecond spacers 222. The second spacers 222 expose the top surface 181 ofthe epitaxial structure 180 while the remaining first mask layer 220covers the top surface 171 of the epitaxial structure 170.

An epitaxial layer 240 is then formed above the top surface 181 of theepitaxial structure 180. The epitaxial layer 240 includes a singleelement semiconductor material, such as germanium (Ge) or silicon (Si),compound semiconductor materials, such as gallium arsenide (GaAs) oraluminum gallium arsenide (AlGaAs), or a semiconductor alloy, such assilicon germanium (SiGe) or gallium arsenide phosphide (GaAsP). In someembodiments, where a P-type device is desired, the epitaxial layer 240may include an epitaxially grown silicon germanium (SiGe). The epitaxialprocesses include CVD deposition techniques (e.g., vapor-phase epitaxy(VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy,and/or other suitable processes. Desired p-type impurities may be, ormay not be, doped while the epitaxial process. The doping may beachieved by an ion implantation process, plasma immersion ionimplantation (PIII) process, gas and/or solid source diffusion process,other suitable process, or combinations thereof. In some embodiments,the epitaxial layer 240 may have a material substantially the same asthe second epitaxial layer 184 of the epitaxial structure 180. In someembodiments, a thickness T3 of the epitaxial layer 240 is about 5 nm toabout 10 nm. If the thickness T3 of the epitaxial layer 240 is less thanabout 5 nm, the epitaxial layer 240 may be totally consumed during theformation of the first metal alloy layer 285 (see FIG. 22); if thethickness T3 of the epitaxial layer 240 is greater than about 10 nm, aresistance between a contact 290 b (see FIGS. 23A and 23B) and theepitaxial structure 180 is increased.

Reference is made to FIG. 16. The remaining first mask layer 220 and thesecond spacers 222 (see FIG. 15) are removed using an etching process,such that the sidewalls of the ILD 195 and the top surface 171 of theepitaxial structure 170 are exposed. On the other hand, since portionsof the second spacers 222 are sandwiched between the CESL 190 and theepitaxial layer 240, these portions of the second spacers 222 may not beremoved and form (dielectric) residues 224 between the CESL 190 (or theILD 195) and the epitaxial layer 240. Hence, the epitaxial layer 240 isspaced apart from the CESL 190 and the ILD 195. In some embodiments, theresidues 224 have a thickness T1′ in a range of about 0.5 nm to about4.5 nm.

Reference is made to FIG. 17. A third mask layer 250 is formed above thestructure of FIG. 16. That is, the third mask layer 250 lines the innersidewalls of the CESL 190 and ILD 195, the top surface 242 of theepitaxial layer 240, and the top surface 171 of the epitaxial structure170. In some embodiments, the third mask layer 250 is a dielectric layersuch as a metal oxide layer. In some embodiments, the third mask layer250 includes a high-k material (k is greater than 7) such as aluminumoxide (Al₂O₃), zirconium oxide (ZrO₂, Zr₂O₃), hafnium oxide (HfO₂),lanthanum oxide (La₂O₃), hafnium aluminum oxide (HfAlO₂), hafniumsilicon oxide (HfSiO₂), or other suitable materials. In someembodiments, the third mask layer 250 may be formed by performing an ALDprocess or other suitable process. In some embodiments, the third masklayer 250 has a thickness T4 in a range of about 3 nm to about 5 nm. Ifthe thickness T4 of the third mask layer 250 is greater than about 5 nm,the deposition window for the following formed epitaxial layer 270 (seeFIG. 20) may be small; if the thickness T4 of the third mask layer 250is less than about 3 nm, the third mask layer 250 may be completelyremoved during the surface cleaning process described in FIG. 20, andthe top surface 242 of the epitaxial layer 240 is exposed.

Subsequently, a fourth mask layer 260 is optionally formed above thethird mask layer 250. The third and fourth mask layers 250 and 260include different materials. For example, the fourth mask layer 260 isfree from metal. The fourth mask layer 260 may be a low-k dielectriclayer, such as silicon oxide, silicon nitride, silicon oxynitride, orcombinations thereof. In some embodiments, the fourth mask layer 260 maybe formed by performing an ALD process or other suitable process. Insome embodiments, the fourth mask layer 260 has a thickness T5 in arange of about 3 nm to about 5 nm. If the thickness T5 of the fourthmask layer 260 is greater than about 5 nm, the deposition window for thefollowing formed epitaxial layer 270 (see FIG. 20) may be small; if thethickness T5 of the fourth mask layer 260 is less than about 3 nm, thefourth mask layer 260 may not protect the first mask layer 250 duringthe following surface cleaning process as described in FIG. 20.

A second photoresist layer 320 is then formed above the P-type region110 p of the substrate 110. The second photoresist layer 320 is formedby spin-on coating or other suitable technique. Other operations, suchas baking, may follow the coating of the second photoresist layer 320.The second photoresist layer 320 exposes a portion of the fourth masklayer 260 above the N-type region 110 n.

Reference is made to FIG. 18. A directional etching process is performedon the portion of the fourth mask layer 260 above the N-type region 110n to remove the horizontal portions of the fourth mask layer 260. Thedirectional etching process may have suitable etch selectivity betweenmaterials of the third and fourth mask layers 250 and 260. For example,the etch process can have a higher etch rate of the fourth mask layer260 than the etch rate of the third mask layer 250. In some embodiments,the etching selectivity of the third mask layer 250 to the fourth masklayer 260 is greater than about 10, e.g., in a range of about 10 toabout 50. In some embodiments, etching rate difference be achieved byadjusting suitable parameters of the etch process such as, for example,etchant gas type, gas flow rate, etching temperature, plasma power,chamber pressure, other suitable parameters, and/or combinationsthereof. The remaining vertical portions of the fourth mask layer 260above the N-type region 110 n form third spacers 262 on the sidewalls ofthe opening 197. Further, a portion of the third mask layer 250 abovethe top surface of the epitaxial structure 170 is exposed by the thirdspacers 262.

Reference is made to FIG. 19. After the formation of the third spacers262, the second photoresist layer 320 (see FIG. 18) is removed from theP-type region 110 p of the substrate 110 by an appropriate process, suchas etching or ashing process. In some embodiments, the fourth mask layer260 is configured to be a protection layer for protecting the third masklayer 250 from being removed during the removal of the secondphotoresist layer 320. In some other embodiments, the fourth mask layer260 can be omitted if the removal process us an etching process that hassuitable etch selectivity between materials of the third mask layer 250and the second photoresist layer 320.

Subsequently, a portion of the third mask layer 250 exposed by thefourth mask layer 260 or the third spacers 262 is removed by using thefourth mask layer 260 and the third spacers 262 as etching masks. Theremoval process may be an etching process that has suitable etchselectivity between materials of the third and fourth mask layers 250and 260 and the ILD 195. For example, the etch process can have a higheretch rate of the third mask layer 250 than the etch rate of the fourthmask layer 260 and the etch rate of ILD 195. As such, the etch processdoes not damage the ILD 195 (and the CESL 190). In some embodiments,etching rate difference be achieved by adjusting suitable parameters ofthe etch process such as, for example, etchant gas type, gas flow rate,etching temperature, plasma power, chamber pressure, other suitableparameters, and/or combinations thereof. As such, the top surface 171 ofthe epitaxial structure 170 is exposed by the third mask layer 250.Further, the remaining vertical portions of the third mask layer 250above the N-type region 110 n form fourth spacers 252 on the sidewallsof the opening 197.

Reference is made to FIG. 20. Another surface cleaning process isperformed on the structure of FIG. 19 for removing native oxides on thetop surface 171 of the epitaxial structure 170. The fourth mask layer260 and the third spacers 262 (see FIG. 19) are also removed during thissurface cleaning process. In some other embodiments, the fourth masklayer 260 can be omitted if the surface cleaning process barely removesthe third mask layer 250 and the fourth spacers 252. As such, theremaining third mask layer 250 and the fourth spacers 252 are exposed.In some embodiments, the surface cleaning process removes portions ofthe remaining third mask layer 250 and the fourth spacers 252, such thatthe thicknesses of the remaining third mask layer 250 and the fourthspacers 252 are reduced. In some other embodiments, the fourth masklayer 260 can be omitted if the surface cleaning process barely removesthe third mask layer 250 and the fourth spacers 252. The fourth spacers252 expose the top surface 171 of the epitaxial structure 170 while theremaining third mask layer 250 covers the top surface 242 of theepitaxial layer 240.

An epitaxial layer 270 is then formed above the top surface 171 of theepitaxial structure 170. The epitaxial layer 270 includes a singleelement semiconductor material, such as germanium (Ge) or silicon (Si),compound semiconductor materials, such as gallium arsenide (GaAs) oraluminum gallium arsenide (AlGaAs), or a semiconductor alloy, such assilicon germanium (SiGe) or gallium arsenide phosphide (GaAsP). In someembodiments, where an N-type device is desired, the epitaxial structures170 may include an epitaxially grown silicon phosphorus (SiP) or siliconcarbon (SiC). The epitaxial processes include CVD deposition techniques(e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD(UHV-CVD)), molecular beam epitaxy, and/or other suitable processes.Desired n-type impurities may be, or may not be, doped while theepitaxial process. The doping may be achieved by an ion implantationprocess, plasma immersion ion implantation (PIII) process, gas and/orsolid source diffusion process, other suitable process, or combinationsthereof. In some embodiments, the epitaxial layer 270 may have amaterial substantially the same as the second epitaxial layer 174 of theepitaxial structure 170. In some embodiments, a thickness T6 of theepitaxial layer 270 is about 5 nm to about 10 nm. If the thickness T6 ofthe epitaxial layer 270 is less than about 5 nm, the epitaxial layer 270may be totally consumed during the formation of the second metal alloylayer 280 (see FIG. 22); if the thickness T6 of the epitaxial layer 270is greater than about 10 nm, a resistance between a contact 290 a (seeFIGS. 23A and 23B) and the epitaxial structure 170 is increased.

Reference is made to FIG. 21. The remaining third mask layer 250 and thefourth spacers 252 (see FIG. 20) are removed using an etching process,such that the sidewalls of the ILD 195 and the top surface 242 of theepitaxial layer 240 are exposed. On the other hand, since portions ofthe fourth spacers 252 are sandwiched between the CESL 190 and theepitaxial layer 270, these portions of the fourth spacers 252 may not beremoved and form (dielectric) residues 254 between the CESL 190 (or theILD 195) and the epitaxial layer 270. Hence, the epitaxial layer 270 isspaced apart from the CESL 190 and the ILD 195. In some embodiments, theresidues 254 have a thickness T4′ in a range of about 0.5 nm to about4.5 nm.

Reference is made to FIG. 22. A first metal alloy layer 280 and a secondmetal alloy layer 285 are respectively formed above the epitaxial layer270 and the epitaxial layer 240. The first metal alloy layer 280 and thesecond metal alloy layer 285, which may be silicide layers, arerespectively formed in the openings 197 and 198 and over the exposedepitaxial layer 270 and the exposed epitaxial layer 240 by aself-aligned silicide (salicide) process. The silicide process convertsthe surface portions of the epitaxial layer 270 and the epitaxial layer240 into the silicide contacts. Silicide processing involves depositionof a metal that undergoes a silicidation reaction with silicon (Si). Inorder to form silicide contacts on the epitaxial layer 270 and theepitaxial layer 240, a metal material is blanket deposited on theepitaxial layer 270 and the epitaxial layer 240. After heating the waferto a temperature at which the metal reacts with the silicon of theepitaxial layer 270 and the epitaxial layer 240 to form contacts,unreacted metal is removed. The silicide contacts remain over theepitaxial layer 270 and the epitaxial layer 240, while unreacted metalis removed from other areas. The silicide layer may include a materialselected from titanium silicide, cobalt silicide, nickel silicide,platinum silicide, nickel platinum silicide, erbium silicide, palladiumsilicide, combinations thereof, or other suitable materials. In someembodiments, the first metal alloy layer 280 and the second metal alloylayer 285 may include germanium.

In some embodiments, after the formation of the first metal alloy layer280 and the second metal alloy layer 285, portions of the epitaxiallayer 270 and the epitaxial layer 240 are consumed such that thethickness thereof are reduced. In some embodiments, each of the firstmetal alloy layer 280 and the second metal alloy layer 285 has athickness T3′ (T6′) in a range of about 1 nm to about 2 nm. If thethickness T3′ (T6′) is lower than about 1 nm or greater than about 2 nm,a resistance between a contact 290 b (290 a) (see FIGS. 23A and 23B) andthe epitaxial structure 180 (170) is increased.

Reference is made to FIGS. 23A and 23B, where FIG. 23B is a side view ofthe semiconductor device of FIG. 23A. Barrier layers 292 a and 292 b arerespectively formed on the first metal alloy layer 280 and the secondmetal alloy layer 285 and respectively in the openings 197 and 198. Thebarrier layer 292 a (292 b) can improve the adhesion between the firstmetal alloy layer 280 (the second metal alloy layer 285) and a materialformed thereon (such as filling material 294 a (294 b)). The barrierlayers 292 a and 292 b may include metal nitride materials. For example,the barrier layers 292 a and 292 b include Ti, TiN, or combinationthereof. In some embodiments, the barrier layers 292 a and 292 b includea single layer or multiple layers. For a multiple-layer configuration,the layers include different compositions of metal nitride from eachother. For example, the barrier layers 292 a and 292 b has a metal layerincluding Ti and a metal nitride layer including TiN.

Filling materials 294 a and 294 b are respectively formed in theopenings 197 and 198 and over the barrier layers 292 a and 292 b. Thefilling materials 294 a and 294 b are electrically connected to theepitaxy structures 170 and 180. In some embodiments, metal materials canbe filled in the openings, and excessive portions of the metal materialsand the barrier layer are removed by performing a CMP process to formthe filling materials 294 a and 294 b and the barrier layer 292 a and292 b. The filling materials 294 a and 294 b can be made of tungsten,aluminum, copper, or other suitable materials. The filling material 294a (294 b) and the barrier layer 292 a (292 b) are referred to as contact290 a (290 b).

In FIGS. 23A and 23B, the semiconductor device includes transistors 100a and 100 b. The transistor 100 a is an N-type transistor, and thetransistor 100 b is a P-type transistor. The transistor 100 a includesthe epitaxial structures 170 serve as source/drain features of thetransistor 100 a. The transistor 100 a further includes isolationstructure (i.e., the CESL 190 and the ILD 195) above the epitaxialstructures 170. The CESL 190 is conformal to the sidewalls of theepitaxial structures 170, and the ILD 195 is above the CESL 190. In someembodiments, some of the bottom air gaps 192 are defined by the CESL 190and formed under the epitaxial structure 170.

The epitaxial layer 270 is above the epitaxial structure 170, and thefirst metal alloy layer 280 is above the epitaxial layer 270. Theepitaxial layer 270 and the first metal alloy layer 280 are both spacedapart from the CESL 190. For example, residues 254 are between and indirect contact with the epitaxial layer 270 and the CESL 190 and alsobetween the first metal alloy layer 280 and the CESL 190. In someembodiments, a topmost surface 254 t of the residue 254 is higher than atopmost surface 280 t of the first metal alloy layer 280. In otherwords, the residue 254 protrudes from the first metal alloy layer 280.

The transistor 100 a further includes the contact 290 a. The barrierlayer 292 a of the contact 290 a is in direct contact with the firstmetal alloy layer 280 and the residues 254. In some embodiments, topportions of the residues 254 are embedded in the barrier layer 292 a. Insome embodiments, a width of the contact 290 a is greater than a widthof the first metal alloy layer 280. The width of the contact 290 a isalso greater than a width of the epitaxial layer 270. The width of thecontact 290 a is less than a width of the second epitaxial layer 174 ofthe epitaxial structure 170. Further, the top surface 171 of theepitaxial structure 170 is wider than a bottom surface 274 of theepitaxial layer 270.

Similarly, the transistor 100 b includes the epitaxial structures 180serve as source/drain features of the transistor 100 b. The isolationstructure (i.e., the CESL 190 and the ILD 195) are above the epitaxialstructures 180. The CESL 190 is conformal to the sidewalls of theepitaxial structures 180, and the ILD 195 is above the CESL 190. In someembodiments, the some of the bottom air gaps 192 are defined by the CESL190 and formed under the epitaxial structure 180.

The epitaxial layer 240 is above the epitaxial structure 180, and thesecond metal alloy layer 285 is above the epitaxial layer 270. Theepitaxial layer 240 and the second metal alloy layer 285 are both spacedapart from the CESL 190. For example, residues 224 are between and indirect contact with the epitaxial layer 240 and the CESL 190 and alsobetween the second metal alloy layer 285 and the CESL 190. In someembodiments, a topmost surface 224 t of the residue 224 is higher than atopmost surface 285 t of the second metal alloy layer 285. In otherwords, the residue 224 protrudes from the second metal alloy layer 285.

The transistor 100 b further includes the contact 290 b. The barrierlayer 292 b of the contact 290 b is in direct contact with the secondmetal alloy layer 285 and the residues 224. In some embodiments, topportions of the residues 224 are embedded in the barrier layer 292 b. Insome embodiments, a width of the contact 290 b is greater than a widthof the second metal alloy layer 285. The width of the contact 290 b isalso greater than a width of the epitaxial layer 240. The width of thecontact 290 b is less than a width of the second epitaxial layer 184 ofthe epitaxial structure 180. Further, the top surface 181 of theepitaxial structure 180 is wider than a bottom surface 244 of theepitaxial layer 240.

The epitaxial structures 170 and 180 are high activations and have highdoping concentrations during the operations in FIG. 8. However, somehigh-temperature processes, such as the formation of the ILD 195, reducethe doping concentrations of the epitaxial structures 170 and 180 alsolower the activations thereof. As such, the contact resistance may beincreased if the contacts 290 a and 290 b are in direct contact with theepitaxial structures 170 and 180. In FIGS. 23A and 23B, the epitaxiallayers 240 and 270 remain high activations and have high dopingconcentrations since they are formed after the high-temperatureprocesses. The epitaxial layers 240 and 270 can improve the contactresistance between the contact 290 a (290 b) and the epitaxialstructures 170 (180).

FIGS. 24 and 25 are side views of semiconductor devices in accordancewith some embodiments of the present disclosure. The difference betweenthe semiconductor devices in FIGS. 24, 25, and 23B pertains to thepresence of the residues 224 and/or 254. In some embodiments, as shownin FIG. 24, the second spacers 222 (see FIG. 15) and/or the fourthspacers 252 (see FIG. 20) are completely removed, such that top air gaps226 are formed on the epitaxial structure 180 and/or top air gaps 256are formed on the epitaxial structure 170. In still some otherembodiments, as shown in FIG. 25, top air gaps 226 (and/or 256) areformed above the residues 224 (and/or 254). Other relevant structuraldetails of the semiconductor devices in FIGS. 24 and 25 are all the sameas or similar to the semiconductor devices in FIGS. 23A and 23B, and,therefore, a description in this regard will not be repeatedhereinafter.

FIGS. 26-36B illustrate a method for manufacturing a semiconductordevice at various stages in accordance with some embodiments of thepresent disclosure. In some embodiments, the semiconductor device shownin FIGS. 26-36B may be intermediate devices fabricated during processingof an integrated circuit (IC), or a portion thereof, that may includestatic random access memory (SRAM), logic circuits, passive componentsand/or active components, such as p-type field effect transistors(PFETs), n-type FETs (NFETs), multi-gate FETs, metal-oxide semiconductorfield effect transistors (MOSFETs), complementary metal-oxidesemiconductor (CMOS) transistors, bipolar transistors, high voltagetransistors, high frequency transistors, other memory cells, andcombinations thereof.

Reference is made to FIG. 26. A substrate 410, which may be a part of awafer, is provided. The substrate 410 includes at least one N-typeregion 410 n and at least one P-type region 410 p. At least one N-typedevice will be formed on the N-type region 410 n, and at least oneP-type device will be formed on the P-type region 410 p. For ease ofexplanation, it is assumed that in FIGS. 26-36B, the substrate 410includes one N-type region 410 n and one P-type region 410 p adjacentthe N-type region 410 n. In some embodiments, the substrate 410 mayinclude silicon (Si). Alternatively, the substrate 410 may includegermanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs) orother appropriate semiconductor materials. In some embodiments, thesubstrate 410 may include a semiconductor-on-insulator (SOI) structuresuch as a buried dielectric layer. Also alternatively, the substrate 410may include a buried dielectric layer such as a buried oxide (BOX)layer, such as that formed by a method referred to as separation byimplantation of oxygen (SIMOX) technology, wafer bonding, SEG, oranother appropriate method. In various embodiments, the substrate 410may include any of a variety of substrate structures and materials.

A stacked structure 415 is formed on the substrate 410 through epitaxy,such that the stacked structure 415 forms crystalline layers. Thestacked structure 415 includes first semiconductor layers 416 and secondsemiconductor layers 418 stacked alternately. The first semiconductorlayers 416 and the second semiconductor layers 418 are made of materialshaving different lattice constants, and may include one or more layersof Si, Ge, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb orInP. In some embodiments, the first semiconductor layers 416 and thesecond semiconductor layers 418 are made of Si, a Si compound, SiGe, Geor a Ge compound. In FIG. 26, five layers of the first semiconductorlayer 416 and five layers of the second semiconductor layer 418 aredisposed. However, the number of the layers are not limited to five, andmay be as small as 1 (each layer) and in some embodiments, 2-10 layersof each of the first and second semiconductor layers are formed. Byadjusting the numbers of the stacked layers, a driving current of theGAA FET device can be adjusted.

In some embodiments, the first semiconductor layers 416 can be SiGelayers having a germanium atomic percentage greater than zero. In someembodiments, the germanium percentage of the first semiconductor layers416 is in the range between about 20 percent and about 30 percent. Insome embodiments, the thickness of the first semiconductor layers 416 isin the range between about 10 nm and about 20 nm.

In some embodiments, the second semiconductor layers 418 may be puresilicon layers that are free from germanium. The second semiconductorlayers 418 may also be substantially pure silicon layers, for example,with a germanium atomic percentage lower than about 1 percent.Furthermore, the second semiconductor layers 418 may be intrinsic, whichare not doped with p-type and n-type impurities. In some embodiments,the thickness of the second semiconductor layers 418 is in the rangebetween about 10 nm and about 20 nm.

Reference is made to FIG. 27. The stacked structure 415 (see FIG. 26) ispatterned into fin structures 432 and 434 and trenches 402. The finstructures 432 and 434 may serve as active regions (e.g., channels andsource/drain features) of transistors. The number of the fin structures432 and 434 is not limited to, and may be as small as one and three ormore. In some embodiments, one or more dummy fin structures are formedon both sides of the fin structures 432 and 434 to improve patternfidelity in the patterning operations.

The trenches 402 extend into the substrate 410, and have lengthwisedirections substantially parallel to each other. The trenches 402 formbase portions 412 and 414 in the substrate 410, where the base portions412 and 414 protrude from the substrate 410, and the fin structures 432and 434 are respectively formed above the base portions 412 and 414 ofthe substrate 410.

Spacing layer material 420′ is deposited on the exposed sidewalls andtop planar surfaces of the fin structures 432 and 434. In someembodiments, the spacing layer material 420′ includes a liner layer 422and a second liner layer 424. The spacing layer material 420′ can bedeposited between the fin structures 432 and 434 to form openings 428,in accordance with some embodiments. The manufacturing processes and/ormaterials of the spacing layer material 420′ is similar to or the sameas the spacing layer material 120′ shown in FIG. 2, and, therefore, adescription in this regard will not be repeated hereinafter.

Subsequently, Self-aligned isolation fins 430 are then formed in theopenings 428. In some embodiments, forming the self-aligned isolationfins 430 further includes performing a planarization step (e.g., a CMPstep) to remove the excess dielectric materials on the upper surfaces ofspacing layer material 420′, so that the upper surfaces of theself-aligned isolation fins 430 and the upper surface of spacing layermaterial 420′ are substantially coplanar. The manufacturing processesand/or materials of the self-aligned isolation fins 430 is similar to orthe same as the self-aligned isolation fins 130 shown in FIG. 3, and,therefore, a description in this regard will not be repeatedhereinafter.

Reference is made to FIG. 28. A planarization process, e.g., a CMPprocess, is performed on the structure of FIG. 27 to expose the finstructures 432 and 434. The spacing layer material 420′ (see FIG. 27) isthen etched back such that portions of the fin structures 432 and 434and the self-aligned isolation fins 430 protrude from the remainingportions of the spacing layer material 420′. The remaining portions ofspacing layer material 420′ forms spacing layer 420. The manufacturingprocesses of the spacing layer 420 are similar to or the same as thespacing layer 120 shown in FIG. 4, and, therefore, a description in thisregard will not be repeated hereinafter.

Subsequently, a sacrificial gate dielectric layer 440 is conformallyformed above the fin structures 432 and 434, the self-aligned isolationfins 430, and the spacing layer 420. At least one dummy gate structure450 is formed above the sacrificial gate dielectric layer 440. The dummygate structure 450 includes a dummy gate layer 452, a pad layer 454formed over the dummy gate layer 452, and a mask layer 456 formed overthe pad layer 454. The manufacturing processes of the sacrificial gatedielectric layer 440 and the dummy gate structure 450 are similar to orthe same as the sacrificial gate dielectric layer 140 and the dummy gatestructure 150 shown in FIG. 5, and, therefore, a description in thisregard will not be repeated hereinafter.

Reference is made to FIG. 29. Gate spacers 460 a are formed on oppositesidewalls of the dummy gate structure 450 and fin sidewall spacers 460 bare formed on opposite sidewalls of the fin structures 432 and 434. Themanufacturing processes of the gate spacers 460 a and the fin sidewallspacers 460 b are similar to or the same as the gate spacers 160 a andthe fin sidewall spacers 160 b shown in FIG. 7, and, therefore, adescription in this regard will not be repeated hereinafter.

Reference is made to FIG. 30. The exposed portions of the fin structures432 and 434 are removed by using a strained source/drain (SSD) etchingprocess. The SSD etching process may be performed in a variety of ways.In some embodiments, the SSD etching process may be performed by a drychemical etch with a plasma source and a reaction gas. The plasma sourcemay be an inductively coupled plasma (ICR) etch, a transformer coupledplasma (TCP) etch, an electron cyclotron resonance (ECR) etch, areactive ion etch (RIE), or the like and the reaction gas may be afluorine-based gas (such as SF₆, CH₂F₂, CH₃F, CHF₃, or the like),chloride (Cl₂), hydrogen bromide (HBr), oxygen (O₂), the like, orcombinations thereof. In some other embodiments, the SSD etching processmay be performed by a wet chemical etch, such as ammonium peroxidemixture (APM), NH₄OH, TMAH, combinations thereof, or the like. In yetsome other embodiments, the SSD etch step may be performed by acombination of a dry chemical etch and a wet chemical etch.

Subsequently, the first semiconductor layers 416 are horizontallyrecessed (etched) so that the second semiconductor layers 418 laterallyextend past opposite end surfaces of the first semiconductor layers 416.In some embodiments, end surfaces of the first semiconductor layers 416may be substantially vertically aligned with the side surfaces of thesacrificial gate electrode layer 440.

Reference is made to FIG. 31. After the first semiconductor layers 416(see FIG. 30) are horizontally recessed, inner spacers 405 are formed onthe recessed surfaces of the first semiconductor layers 416, as shown inFIG. 31. Formation of the inner spacer 405 includes depositing an innerspacer material layer (e.g., silicon nitride), followed by etching backthe inner spacer material layer by an anisotropic etching process, toremove the inner spacer material layer from the substrate 410. In someembodiments, the inner spacers 405 include insulating material such assilicon nitride or the like. The thickness of the inner spacer 405 onthe recessed surface of the first semiconductor layers 412 is in a rangefrom about 5 nm to about 10 nm, in some embodiments.

Reference is made to FIG. 32. Epitaxial structures 470 and 480, whichare referred to as source/drain regions, are epitaxially grown from theexposed recessed fin structures 432 and 434 (see FIG. 31) between thefin sidewall spacers 460 b. In some embodiments, the epitaxial structure470 includes a first epitaxial layer 472 and a second epitaxial layer474, and the epitaxial structure 480 includes a first epitaxial layer482 and a second epitaxial layer 484. The manufacturing processes of theepitaxial structures 470 and 480 are similar to or the same as theepitaxial structures 170 and 180 shown in FIG. 8, and, therefore, adescription in this regard will not be repeated hereinafter.

A contact etch stop layer (CESL) 490 is conformally formed over theepitaxial structures 470 and 480, and an interlayer dielectric (ILD) 495is then formed on the CESL 490. The manufacturing processes of the CESL490 and the ILD 495 are similar to or the same as the CESL 490 and theILD 495 shown in FIG. 9, and, therefore, a description in this regardwill not be repeated hereinafter.

Subsequently, a replacement gate (RPG) process scheme is employed. Thedummy gate structure 450 (see FIG. 31) and the first semiconductorlayers 416 (see FIG. 30) are replaced with a metal gate structure 510.The metal gate structure 510 includes a gate dielectric layer 512 and ametal gate electrode 514 over the gate dielectric layer 512. The metalgate electrode 514 may include metal layers 515, e.g., work functionmetal layer(s) and capping layer(s), a fill layer(s) 516, and/or othersuitable layers that are desirable in a metal gate stack. Themanufacturing processes of the metal gate structure 510 are similar toor the same as the metal gate structure 210 shown in FIG. 10, and,therefore, a description in this regard will not be repeatedhereinafter.

Reference is made to FIGS. 33A and 33B, where FIG. 33B is a side view ofthe semiconductor device in FIG. 33A. The ILD 495 and the CESL 490 arethen etched to form openings 497 and 498. The manufacturing processes ofthe openings 497 and 498 are similar to or the same as the openings 197and 198 shown in FIGS. 11A and 11B, and, therefore, a description inthis regard will not be repeated hereinafter.

Reference is made to FIG. 34. An epitaxial layer 540 is formed above thetop surface 481 of the epitaxial structure 480. In some embodiments, theformation of the epitaxial layer 540 is similar to the formation of theepitaxial layer 240 shown in FIG. 16. For example, similar to FIGS.12-16, a first mask layer and a second mask layer are formed in theopenings 497 and 498. The second mask layer is patterned to form firstspacers on sidewalls of the opening 498. The first mask layer ispatterned using the first spacers as etching masks to form secondspacers on the sidewalls of the opening 498, such that the top surface481 of the epitaxial structure 480 is exposed. The first spacers areremoved, and the epitaxial layer 540 is formed above the epitaxialstructure 480. The second spacers are then removed. In some embodiments,residues 524 are formed between the epitaxial layers 540 and theisolation structure (i.e., the CESL 190 and/or the ILD 195) as shown inFIG. 34. In some other embodiments, the second spacers are completelyremoved, such that top air gaps are formed between the epitaxial layers540 and the isolation structure (i.e., the CESL 190 and/or the ILD 195)similar to the structure shown in FIG. 24. In still some otherembodiments, top air gaps are formed above the residues 524 similar tothe structure shown in FIG. 25. Materials, configurations, dimensions,processes and/or operations regarding the epitaxial layer 540 aresimilar to or the same as the epitaxial layer 240 of FIG. 16.

Reference is made to FIG. 35. An epitaxial layer 570 is formed above thetop surface 471 of the epitaxial structure 470. In some embodiments, theformation of the epitaxial layer 570 is similar to the formation of theepitaxial layer 270 shown in FIG. 21. For example, similar to FIGS.17-21, a third mask layer and a fourth mask layer are formed in theopenings 497 and 498. The fourth mask layer is patterned to form thirdspacers on sidewalls of the opening 497. The third mask layer ispatterned using the third spacers as etching masks to form fourthspacers on the sidewalls of the opening 497, such that the top surface471 of the epitaxial structure 470 is exposed. The third spacers areremoved, and the epitaxial layer 570 is formed above the epitaxialstructure 470. The fourth spacers are then removed. In some embodiments,residues 554 are formed between the epitaxial layers 570 and theisolation structure (i.e., the CESL 190 and/or the ILD 195) as shown inFIG. 35. In some other embodiments, the fourth spacers are completelyremoved, such that top air gaps are formed between the epitaxial layers570 and the isolation structure (i.e., the CESL 190 and/or the ILD 195)similar to the structure shown in FIG. 24. In still some otherembodiments, top air gaps are formed above the residues 554 similar tothe structure shown in FIG. 25. Materials, configurations, dimensions,processes and/or operations regarding the epitaxial layer 570 aresimilar to or the same as the epitaxial layer 270 of FIG. 21.

Reference is made to FIGS. 36A and 36B, where FIG. 36B is a side view ofthe semiconductor device in FIG. 36A. A first metal alloy layer 580 anda second metal alloy layer 585 are respectively formed above theepitaxial layer 570 and the epitaxial layer 540. The manufacturingprocesses of the first metal alloy layer 580 and the second metal alloylayer 585 are similar to or the same as the first metal alloy layer 280and the second metal alloy layer 285 shown in FIG. 22, and, therefore, adescription in this regard will not be repeated hereinafter.

Contacts 590 a and 590 b are then respectively formed above the firstmetal alloy layer 580 and the second metal alloy layer 585. The contact590 a includes a barrier layer 292 a and a filling material 294 a, andthe contact 590 b includes a barrier layer 292 b and a filling material294 b. As such, the semiconductor device includes (HGAA) transistors 400a and 400 b. The manufacturing processes of the contacts 590 a and 590 bare similar to or the same as the contacts 290 a and 290 b shown inFIGS. 23A and 23B, and, therefore, a description in this regard will notbe repeated hereinafter.

FIG. 37 is a flow chart of a method M for forming a semiconductor devicein accordance with some embodiments of the present disclosure. Althoughthe method M is illustrated and/or described as a series of acts orevents, it will be appreciated that the method is not limited to theillustrated ordering or acts. Thus, in some embodiments, the acts may becarried out in different orders than illustrated, and/or may be carriedout concurrently. Further, in some embodiments, the illustrated acts orevents may be subdivided into multiple acts or events, which may becarried out at separate times or concurrently with other acts orsub-acts. In some embodiments, some illustrated acts or events may beomitted, and other un-illustrated acts or events may be included.

At block S12, a fin structure is formed above a substrate. In someembodiments, the fin structure is a semiconductor fin as shown inFIG. 1. In some other embodiments, the fin structure includes first andsecond semiconductor layers alternately stacked as shown in FIG. 27.FIGS. 1 and 27 illustrate perspective views of some embodimentscorresponding to act in block S12. At block S14, a gate structure isformed above the fin structure. FIGS. 5 and 28 illustrate perspectiveviews of some embodiments corresponding to act in block S14. At blockS16, an epitaxial structure is formed on a side of the gate structure.FIGS. 8 and 32 illustrate perspective views of some embodimentscorresponding to act in block S16. At block S18, an isolation structureis formed above the epitaxial structure. In some embodiments, theisolation structure includes a CESL and/or an ILD. FIGS. 9 and 32illustrate perspective views of some embodiments corresponding to act inblock S18. At block S20, an opening is formed in the isolation structureto expose the epitaxial structure. FIGS. 11A and 33A illustrateperspective views of some embodiments corresponding to act in block S20.At block S22, a mask layer is formed in the opening. FIGS. 12 and 17illustrate side views of some embodiments corresponding to act in blockS22. At block S24, the mask layer is patterned to form a spacer onsidewalls of the opening, such that a top surface of the epitaxialstructure is exposed. FIGS. 14 and 19 illustrate side views of someembodiments corresponding to act in block S24. At block S26, anepitaxial layer is form above the top surface of the epitaxialstructure. FIGS. 15 and 20 illustrate side views of some embodimentscorresponding to act in block S26. At block S28, the spacer is removed.FIGS. 16 and 21 illustrate side views of some embodiments correspondingto act in block S28. At block S30, a metal alloy layer is formed abovethe epitaxial layer. FIG. 22 illustrates a side view of some embodimentscorresponding to act in block S30. At block S32, a contact is formedabove the metal alloy layer. FIGS. 23A, 23B, 36A, and 36B illustrateperspective views and side views of some embodiments corresponding toact in block S32.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages. It is understood, however, that otherembodiments may offer additional advantages, and not all advantages arenecessarily disclosed herein, and that no particular advantage isrequired for all embodiments. One advantage is that the epitaxial layers240 and 270 remain high activations and have high doping concentrations,such that the contact resistance between the contact and the epitaxialstructure can be improved. Another advantage is that the mask layer hashigh etch selectivity with respect to the ILD and CESL, the patterningprocess of the mask layer does not damage the ILD and CESL.

According to some embodiments, a device includes an active region, agate structure, an epitaxial structure, an epitaxial layer, a metalalloy layer, a contact, and a contact etch stop layer. The gatestructure is across the active region. The epitaxial structure is abovethe active region and adjacent the gate structure. The epitaxial layeris above the epitaxial structure. The metal alloy layer is above theepitaxial layer. The contact is above the metal alloy layer. The contactetch stop layer lines sidewalls of the epitaxial structure. The metalalloy layer is spaced apart from the contact etch stop layer.

According to some embodiments, a device includes a fin structure, a gatestructure, an epitaxial structure, a metal alloy layer, a contact etchstop layer, and a residue. The gate structure is above the finstructure. The epitaxial structure is above the fin structure. The metalalloy layer is above the epitaxial structure. The contact etch stoplayer is conformal to sidewalls of the epitaxial structure. The residueis above the epitaxial structure and between the metal alloy layer andthe contact etch stop layer.

According to some embodiments, a method includes forming a gatestructure above an active region. An epitaxial structure is formed abovethe active region and adjacent the gate structure. An isolationstructure is formed above the epitaxial structure. An opening is formedin the isolation structure to expose the epitaxial structure. A firstmask layer is formed to line inner sidewalls of the isolation structureand a top surface of the epitaxial structure. The first mask layer ispatterned to form a first spacer layer on the inner sidewalls of theisolation structure. An epitaxial layer is formed above the epitaxialstructure. The first spacer layer is removed. A metal alloy layer isformed above the epitaxial layer. A contact is formed above the metalalloy layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a gate structureabove an active region; forming an epitaxial structure above the activeregion and adjacent the gate structure; forming an isolation structureabove the epitaxial structure; forming an opening in the isolationstructure to expose the epitaxial structure; forming a first mask layerto line inner sidewalls of the isolation structure and a top surface ofthe epitaxial structure; patterning the first mask layer to form a firstspacer layer on the inner sidewalls of the isolation structure; formingan epitaxial layer above the epitaxial structure; removing a firstportion of the first spacer layer, wherein a second portion of the firstspacer layer remains on and extends along sidewalls of the epitaxiallayer; forming a metal alloy layer above the epitaxial layer, wherein atop of the second portion of the first spacer layer is higher than a topof the metal alloy layer; and forming a contact above the metal alloylayer.
 2. The method of claim 1, wherein the first mask layer is ahigh-k dielectric layer.
 3. The method of claim 1, wherein patterningthe first mask layer comprises: forming a second mask layer above thefirst mask layer; patterning the second mask layer to form a secondspacer layer on the inner sidewalls of the isolation structure; andetching the first mask layer using the second spacer layer as an etchingmask to form the first spacer layer.
 4. The method of claim 3, whereinan etching selectivity of the first mask layer to the second mask layeris greater than about
 10. 5. The method of claim 1, wherein the gatestructure extends in a first direction, and a width of the opening inthe first direction is less than a width of the epitaxial structure inthe first direction.
 6. The method of claim 1, wherein the epitaxialstructure is wider than the epitaxial layer.
 7. A method comprising:forming a gate structure over a semiconductor fin; epitaxially growing asource/drain epitaxial structure over the semiconductor fin and adjacentthe gate structure; depositing an interlayer dielectric (ILD) layer overthe source/drain epitaxial structure and surrounding the gate structure;patterning the ILD layer to form an opening in the ILD layer andexposing the source/drain epitaxial structure; depositing a spacer layerin the opening and covering the source/drain epitaxial structure;forming a first spacer on a sidewall of the opening and in contact withthe spacer layer; etching the spacer layer by using the first spacer asan etching mask to form a second spacer between the first spacer and theILD layer; after etching the spacer layer, removing the first spacer;after removing the first spacer, epitaxially growing an epitaxial layerover the source/drain epitaxial structure and in contact with the secondspacer; and forming a contact over the epitaxial layer.
 8. The method ofclaim 7, further comprising etching back the second spacer afterepitaxially growing the epitaxial layer.
 9. The method of claim 7,further comprising forming a metal alloy layer over the epitaxial layer.10. The method of claim 9, wherein after forming the metal alloy layer,a thickness of the epitaxial layer is in a range of about 1 nm to about2 nm.
 11. The method of claim 7, wherein the epitaxial layer and thesource/drain epitaxial structure comprise substantially the samematerial.
 12. The method of claim 7, wherein a dielectric constant ofthe second spacer is greater than a dielectric constant of the firstspacer.
 13. The method of claim 7, further comprising depositing acontact etch stop layer (CESL) over the source/drain epitaxial structureprior to depositing the ILD layer, and the CESL is spaced apart from theepitaxial layer.
 14. A method comprising: forming a gate structure overa first fin structure and a second fin structure; forming a first recessin the first fin structure and a second recess in the second finstructure; depositing a first source/drain epitaxial structure in thefirst recess of the first fin structure; depositing a secondsource/drain epitaxial structure in the second recess of the second finstructure; depositing an interlayer dielectric (ILD) layer over thefirst source/drain epitaxial structure and the second source/drainepitaxial structure; forming first and second openings in the ILD layersuch that the first opening exposes the first source/drain epitaxialstructure and the second opening exposes the second source/drainepitaxial structure; forming a first epitaxial layer over the firstsource/drain epitaxial structure; depositing a first mask layer to coverthe first epitaxial layer and the second source/drain epitaxialstructure; patterning the first mask layer such that the patterned firstmask layer covers the first epitaxial layer and exposes a first portionof a top surface of the second source/drain epitaxial structure, whereina second portion of the top surface of the second source/drain epitaxialstructure is in contact with the patterned first mask layer; afterpatterning the first mask layer, forming a second epitaxial layer overthe second source/drain epitaxial structure; after forming the secondepitaxial layer, removing the patterned first mask layer; and forming afirst contact over the first epitaxial layer and a second contact overthe second epitaxial layer.
 15. The method of claim 14, whereinpatterning the first mask layer comprises: depositing a second masklayer over the first mask layer; patterning the second mask layer suchthat the patterned second mask layer covers the first epitaxial layerand exposes a portion of the first mask layer directly above the secondsource/drain epitaxial structure; and patterning the first mask layer byusing the patterned second mask layer as an etching mask.
 16. The methodof claim 15, wherein a dielectric constant of the first mask layer isgreater than a dielectric constant of the second mask layer.
 17. Themethod of claim 15, wherein the first and second epitaxial layerscomprise different materials.
 18. The method of claim 14, furthercomprising forming a dielectric material on a sidewall of the firstepitaxial layer prior to forming the first mask layer.
 19. The method ofclaim 18, wherein forming the first mask layer is such that the firstmask layer is in contact with the dielectric material.
 20. The method ofclaim 18, wherein forming the first mask layer is such that an air gapis between the first mask layer and the dielectric material.